Type this, well type the stuff after the In[1]:=, into the freshly opened Mathematica window. Then hold down the shift key and press Enter.

In[1]:= f[a_,b_,c_,d_]:=a^2+Log[b]+1/c+(d-1)/2;
f[2.1,3.0,1/4,2]

And it should display

Out[2]= 10.0086

There used to be a really nice Wolfram "getting started with Mathematica" page, but all I find at the moment is a video with flying things far beyond what getting started seems like.

Reading through this might help you
http://www.indiana.edu/~statmath/math/mma/gettingstarted/printable.pdf

Mathematica is FANATIC about correct capitalization and use of [] versus () versus {} and = versus := versus == and there is even an === and those are all different. Make any tiny error in any one of those and you may get incorrect results or error messages you don't know how to understand or even nothing at all.